Aflatoxin B1 (AFB) is a fungal liver toxin and carcinogen that frequently contaminates foodstuffs such as corn and peanuts in some regions of the world. There are large species differences in sensitivity to AFB-induced liver cancer. Expression of specific forms of glutathione S-transferases (GST) plays an important role in determining species sensitivity to AFB. Mice are resistant to AFB because they express an alpha-class GST isoform (mGSTA3-3) that has high activity toward the reactive intermediate, AFB-8,9-epoxide (AFBO). In contrast, rats do not constitutively express a homologous form of GST with high AFBO activity and are thus sensitive to AFB-induced hepatocarcinogenesis. We have found that human liver cytosol has no AFBO detoxifying activity, but liver from a nonhuman primate species, Macaca fascicularis (MF), has significant hepatic AFBO-GST activity. Previous studies that utilized a MF hepatic cDNA library identified three different alpha class GSTs, but none had measurable AFBO activity. To determine which form of GST is responsible for this activity, we used a protein purification scheme (glutathione agarose affinity purification followed by chromatography and chromatofocusing). A fraction of GST with high AFBO activity was isolated and identified as a mu-class GST. Reverse-transcriptase polymerase-chain-reaction (RT-PCR) cDNA cloning was used to identify the specific mu-class GST related to this AFBO detoxification. Two different mu-class MF GST cDNAs were obtained. One of these clones is homologous to human GSTM4 and has no AFBO activity. We have subcloned the other mu-class cDNA into an expression system, and the protein expression and characterization on this GST is in progress. Our results suggest that a mu-class GST is responsible for the hepatic cytosolic GST activity toward AFBO in this species of nonhuman primate.